1. Field of the Invention:
This invention relates to a process for the determination of the concentration of substances dissolved in a solvent by means of an osmotic cell with a wall which is as rigid as possible, the osmotic cell being provided with pressure measurement devices for measurement of the hydrostatic pressure in the osmotic cell and the membrane of the osmotic cell having the highest possible conductivity for the solvent and exhibiting a sufficient retention capacity for the substances, by means of which the osmometer solution in the osmotic cell is brought into communication, through the membrane, with the solution to be tested for the concentration of substances, whereby first, using an appropriate solution placed in communication with the osmometer solution via the membrane as well as an appropriate osmometer solution, a working pressure P.sub.O is established and determined, whereupon, after replacement of the solution by the solution to be tested, the concentration of the impermeable substance or substances in the solution to be tested is determined from the pressure curve established in the osmotic cell.
This invention also relates to an apparatus for the determination of the concentration of substances dissolved in a solvent, the apparatus including at least one vessel and at least one measuring head, the measuring head comprising an osmotic cell with a pressure measurement device, which has a rigid metal or plastic wall and whose membrane exhibits the highest possible modulus of elasticity, the highest possible conductivity for the solvent, and a sufficient retention capacity for the substances in the solution to be tested, whereby the ratio of the volume of the osmotic cell to the effective surface of the membrane is a maximum of 0.2 mm, whereby the measurement head or heads can be immersed in the vessel, and whereby the vessel or containers can be filled with a solution (reference solution) to establish the working pressure P.sub.O, with the solution to be tested and, if necessary, with standard solutions.
2. Description of the Prior Art:
The above-mentioned process and the apparatus are described in U.S. Pat. No. 4,706,495, which corresponds to German Laid Open Patent Appln. No. DE-OS 35 25 668. They are used for the determination of the concentration of substances in a solution, if two or more substances (one permeable in relation to the membrane and one or more impermeable substances) are in the solution. The above-mentioned documents are incorporated herein by reference as if the texts thereof were fully set forth herein.
U.S. Pat. No. 4,706,495, which corresponds to German Laid Open Patent Appln. No. DE-OS 35 25 668 describes the process (Process Ia), in which, after a sufficiently rapid exchange of the solution to be tested and the pure solvent, the pressure curve in the osmotic cell, the minimum pressure P.sub.min established and the subsequent final pressure P.sub.E are measured, whereupon the concentration of the impermeable substance or substances in the solution to be tested is determined on the basis of standard values from the pressure difference P.sub.O - P.sub.E and the concentration of the permeable substance in the solution is determined from the difference P.sub.E - P.sub.min. The process can also be executed so that the concentration of impermeable (C.sub.imp.sup.o) substances and permeable (C.sub.s.sup.o) substance in the solution to be tested is determined by calibration of the measurement system with different solutions containing the impermeable and permeable substances in different, known concentrations, whereby for the determination of the permeable components, care must be taken that the standard solution contains the impermeable substance in approximately the same concentration as the solution to be tested, whereby the concentration of permeable substance is taken from the graph (P.sub.E - P.sub.min)=f(C.sub.s.sup.o) and the concentration of impermeable substances is taken from the graph (P.sub.O - P.sub.E)=g(C.sub.imp.sup.o). (P.sub.O - P.sub.E)=g(C.sub.imp.sup.o) represents a straight line in each case, while (P.sub.E - P.sub.min)=f(C.sub.s.sup.o) can deviate from the linear form, since t.sub.min is a function of the mixing ratio .sigma..sub.s.C.sub.s.sup.o C.sub.imp.sup.o.
U.S. Pat. No. 4,706,495, which corresponds to German Laid Open Patent Appln. No. DE-OS 35 25 668 also describes a process (Process Ib) in which, after the exchange of the solution to be tested and the solvent, the minimum pressure P.sub.min established in the osmotic cell, the time t.sub.min, within which the minimum pressure P.sub.min is established, the rate constant k.sub.s for the exponential pressure increase which occurs following the minimum pressure and the final pressure P.sub.E established after the pressure increase are determined. Then, the concentration C.sub.s.sup.o of the permeable substance s is determined from the equation EQU P.sub.E - P.sub.min =.sigma..sub.s.RT.C.sub.s.sup.o.exp(-k.sub.s.t.sub.min)(1)
where R=8.31434 J/K.degree. mol, EQU T=absolute temperature
and .sigma..sub.s =reflection coefficient of s and the concentration (C.sub.imp.sup.o) of the impermeable substance or substances is determined from the equation EQU P.sub.O - P.sub.E =RT.C.sub.imp.sup.o ( 2)
In the execution of the known processes, the most rigid possible osmotic cell is used, i.e., an osmotic cell with a rigid wall and a rigid membrane, so that during the execution of the process, the change in the volume of the osmotic cell is negligible. Under these conditions, equations 1 and 2 apply.
If, however, the membrane and the osmotic cell are not sufficiently rigid, and the change in the volume of the osmotic cell during the measurement is not negligible, the modulus of volume elasticity .epsilon. is determined by means of an appropriate device (for example, a control rod and a micrometer, and through use of an appropriate formula set forth herein), and the concentration C.sub.s.sup.o of the permeable substance is determined from the equation ##EQU1## while the concentration C.sub.imp.sup.o of the impermeable substance is determined from the equation ##EQU2##
The membrane to be used for the Processes Ia and Ib should exhibit the greatest possible conductivity for the solvent and a sufficient retention capacity for the substances. [As used herein, "retention" capacity refers to the reduced permeability that the membrane of the osmotic cell to the substances to be measured, as opposed to its much greater permeability vis a vis the solvent. Alternatively, the "rejection" capacity of the membrane embraces the same concept.] The retention capacity of the membrane is sufficient for the substances if the exchange of the permeable substance through the membrane is delayed by the membrane in relation to the exchange of the pure solvent through the membrane, so that during the exchange phase of the pure solvent, the exchange of the permeable substance is only slight, and if also the exchange of the impermeable substance through the membrane is delayed in relation to the exchange of the permeable substance, so that during the exchange of the permeable substance, no significant exchange of the impermeagle substance takes place, i.e., if the rate constant k.sub.s for the permeable substance is sufficiently great compared to that of the impermeable substance, i.e., if during the test, practically none of the impermeable substance gets through the membrane. On the other hand, that means that with the specified membrane, a substance is to be considered permeable if, during the execution of the process, it is sufficiently delayed in relation to the pure solvent, but still gets through the membrane relatively rapidly in relation to the impermeable substance. Accordingly, a substance is to be considered impermeable if, during the execution of the process, practically none gets through the membrane. In this sense, accordingly, a material can even be practically impermeable, although the reflection coefficient .sigma..sub.s &lt;1, if its permeability (i.e., k.sub.s) in the membrane is sufficiently low.
In the processes of the prior art, to establish the working pressure P.sub.O in the osmotic cell, a solution is used as an osmometer solution in which there is an impermeable substance, so that as a result of the osmotic pressure difference across the membrane, the hydrostatic working pressure P.sub.O builds up in the osmotic cell.
The exchange of the solution to be tested and the solvent must take place very rapidly in comparison to the half-life for the flow of the pure solvent through the membrane, which can be in the range of seconds.
In the execution of the alternative prior art Process Ib, compared to the first Process Ia, the time t.sub.min must also be determined from the beginning of the exchange process of the two solutions (solvent and solution to be tested) until the establishment of the minimum pressure P.sub.min and the rate constant k.sub.s for the pressure increase which occurs following the minimum pressure, which is a consequence of the permeation of the permeable substance through the membrane. Before the execution of Process Ib, the reflection coefficient .sigma..sub.s, which is a material constant with a given membrane and a given solvent, must also be determined in the manner of the prior art.
A process known as Process II is also described in U.S. Pat. No. 4,706,495, which corresponds to German Laid Open Patent Appln. No. DE-OS 35 25 668, which consists of the Process Variants IIa,and IIb. In this prior art process, after the exchange of the solvent and the solution to be tested, which contains two substances, the initial slope (dP/dt).sub.t=0 of the pressure/time curve is measured from the pressure decrease in the osmotic cell, and with this value, according to calibrated values or from the equation ##EQU3## the result EQU X.sub.1 =.sigma..sub.1.C.sub.1 +.sigma..sub.2.C.sub.2 ( 6)
is determined.
In these equations:
A.sub.o is the effective surface of the membrane,
V.sub.o is the volume of the osmotic cell,
Lp is the hydraulic conductivity of the membrane,
.epsilon. is the modulus of volume elasticity of the osmotic cell,
R=8.31434 J/.degree. K. mol,
T=absolute temperature,
.sigma..sub.1 =reflection coefficient of the substance 1,
C.sub.1 =the concentration of the substance 1,
.sigma..sub.2 =the reflection coefficient of the substance 2, and
C.sub.2 =the concentration of the substance 2.
According to the Process Variant IIa, simultaneous with the determination of the result X.sub.1, according to the process described, another measurement is performed using a second osmotic cell with a membrane having a different retention (or rejection) capacity for the substance, whereby another result EQU X.sub.2 .sigma..sub.1 '.C.sub.1 .sigma..sub.2 '.C.sub.2 ( 7)
is determined, and then, by means of the two results X.sub.1 and X.sub.2, the concentration of the two substances is calculated. According to the Process Variant IIb, the additional measurement is executed by means of the second osmotic cell according to one of the Process Ia or Ib, whereby a membrane is used which has a retention capacity for the substance such that, in relation to the membrane used, the one substance is permeable and the other substance is impermeable, and whereby it is sufficient for only the concentration of the permeable or the concentration of the impermeable substance to be determined, and using the result X.sub.1, the concentration of both substances can be calculated.
For the execution of the Process Variant IIa, according to which the result X.sub.1 and simultaneously the result X.sub.2 are determined, it must be guaranteed that the time constants of the osmotic cells are sufficiently low. "Sufficiently low" means, for example, if a blending process is being observed, that the time constant of the blending process is high compared to that of the mixing process in the osmotic cell. This is the case if the K-values of the osmotic cells ##EQU4## are sufficiently high. This can be achieved by the selection of suitable osmotic cells, since it is a question with the factors A.sub.o /V.sub.o, Lp and .epsilon. of structurally specific values of an osmotic cell or its membrane. On the other hand, the initially linear course of the pressure/time curve which is established as the result of a change in concentration, compared to the duration of the mixing phase which takes place during the exchange of two solutions in the measurement system, must be sufficiently long.
The reflection coefficients .sigma..sub.1 and .sigma..sub.2 must be determined in advance. For the determination of .sigma..sub.1 and .sigma..sub.2, the initial slope caused by substance 1 or 2 of known concentration is compared to that obtained after the addition of a known concentration of an impermeable substance.
In Process Variant IIa, the membranes of the two osmotic cells are selected so that the values for .sigma..sub.1 and .sigma..sub.1 ' and .sigma..sub.2 and .sigma..sub.2 ' differ sufficiently.
The constant value K for a specified osmotic cell is determined by calibration.
The short-term mixing of the solutions which occurs during the exchange of solutions in a vessel or in a pipeline can lead to a situation where the initial pressure decrease in the osmotic cell is imprecisely defined. Only when the solutions have been completely exchanged, is the linear course of the pressure/time curve established, from which then, during the execution of Process II, the initial slope is determined.
The Process Variant IIa makes possible a more rapid determination of the concentrations of the two substances than do Processes Ia and Ib, since in the latter, the path of the entire pressure curve must be watched until the final value P.sub.E is achieved. For this purpose, however, the execution of Process IIa (and Process IIb) requires the use of two osmotic cells; the execution of Process Ia or Ib, however, requires only one osmotic cell for either process.
With Process IIb, too, according to which the simultaneous measurement is made via the determination of the pressure difference (P.sub.E - P.sub.min) or (P.sub.O - P.sub.e), a more rapid determination of the concentration of a substance is possible if the concentration of one of the substances is constant, and only the concentration of the other substance is to be measured.
Of course, even if it is not primarily a question of the determination of the concentration of the substances, but of the determination of the time of the change in concentration in a solution in a vessel or a tube, Process II can be conducted without a simultaneous measurement, i.e., the pressure decrease in the osmotic cell is determined by means of only one osmotic cell. In this manner, one osmotic cell can be used as a control or alarm device.
Applications for the determination of the concentration of substances include, in particular, the determination of low-molecular, permeable substances and high-molecular, impermeable substances. One application, for example, is the determination of the concentration of (permeable) alcohol and (impermeable) sugar during alcoholic fermention. Such a determination is significant both for the production of alcoholic beverages and also for the production of industrial alcohol. The processes of the prior art are also useful for the determination of blood alcohol concentration in human beings.
Other applications are the determination of the concentration of organic solvents in aqueous solutions (e.g., of ethanol methanol, propanel, esters, ethers, acetone, etc.) as they occur in certain chemical processes. Another area of application in the chemical industry is the determination of solvent residues, salts and other pollutants in waste waters.
The processes of the prior art can also be used in the monitoring of dialysis processes.
A special application for the prior art Process II is in the rapid recognition of a change in a solution, such as occurs in a brewery or in other sectors of the food and beverage industry when there is a change between a purification agent (e.g., water) and a product (e.g., beer).
By means of the rapid determination of the time of the solution change at a given point, product losses can be strictly limited. Process II can also be used in connection with only one single osmotic cell as an alarm system, e.g., to detect leaks in tanks, pipelines, etc. on account of its ability to recognize rapid changes in concentration.
Therefore, there are many applications of the known processes. One disadvantage of these known processes, however, is that, for example, high concentrations of substances can only be measured with a relatively limited precision.